1. Field of the Invention
The present invention relates to an optical modulator having the Mach-Zender type and a method to generate modulated light using this modulator.
2. Related Prior Arts
A Japanese Patent published as JP-H09-090301A has disclosed a method to generate an optical signal modulated with a Mach-Zender modulator. The method disclosed therein first divides the input light into two branched light and each branched light enters in the first and second waveguides. The respective waveguides provide an electrode thereon, one of which is coupled with a signal source, while, the other of which is grounded. The waveguide biased by a modulating signal, the phase of the light propagating therein advances or delays; accordingly, the output of the modulator which multiplexes two beams each propagating the first and second waveguides may be switched depending on the bias level applied to the first electrode and a length of the electrode.
The modulator with the arrangement disclosed in the prior patent above causes the phase shift only for the light propagating in the first waveguide, which inevitably causes a drift of the wavelength, namely, the chirping in the multiplexed light. Accordingly, a transmission distance is limited as a matter of course.
Another Japanese Patent Application published as JP-2003-322831A has disclosed an advanced modulator with the Mach-Zender arrangement for eliminating the chirping. The modulator disclosed therein has also two waveguides each providing an electrode. However, this modulator applies the electrical signal to both electrodes but in the differential mode. That is, the first electrode on the first waveguide is driven by the signal, and the second electrode on the second waveguide is driven by the other signal whose phase is opposite to the former signal. Such an arrangement of the Mach-Zender modulator, not only the modulation efficiency but the chirping performance may be enhanced.